Crude gold arrives on the market in ingot form from various sources.
Primary sources are golds recovered from ores by conventional and established methods, such as the amalgamation process by the cyanide process and by the chlorine process. The gold extracted and isolated by these processes is then refined by electrolysis of these golds from chloride or cyanide solutions. The gold recovered at the cathodes is usually deposited on foils. The gold recovered from the cathodes is then melted and cast into the gold ingots. The purity of this commercial ingot gold is about 96%. It contains the usual trace elements associated with gold ores such as copper, silver, arsenic and nickels.
Secondary gold sources are the recycled industrial gold wastes from castings or machine turnings such as spatters, sprues, trimmings, polishings, rejected castings and quality control rejects of electronic parts. Such secondary or recycled industrial golds contain the various alloying metals and elements used by the jewelry and other gold generating trades. These include the various coloralloying metals as well as hardening elements or melt additives for the alloys.
The trace elements and the various alloying metals must be removed before gold can be properly re-used. Various separating and refining methods have been proposed and some are still in active use.
In all cases the primary and secondary golds are melted and cast into "crude ingots". These crude ingots contain from up to 98% gold.
The method for gold refining generally used, called the "mint method" because of their primary user, consists in electrolyzing the crude ingots as gold anodes in hot acidic solutions, initially containing 7-8% gold chloride and 10% hydrochloric acid. Current densities as high as 110 amps/ft.sup.2 of cathode surface are used and the refined gold is electrodeposited unto foil or rolled sheet gold cathodes. The gold deposited is "mint" grade, 98.5% purity. The electrolysis cells used at the mints and assay offices are generally constructed to specification of glazed porcelain or chemical stoneware, During the mint electrolysis, platinum and palladium, often present, remain in solution in the haloacidic electrolyte. The halides in the electrolyte precipitate any silver from the alloys as silver chloride. To prevent reduction of any silver particles in proximity of the cathode onto the cathode, it is the practice to surround the anodes with permeable bags usually of nylon cloth. This anode bagging is practiced to confine the insoluble AgCl away from the cathode. At periodic intervals, because of the accumulation of electrolyte-insoluble "dross" impurities, the bags are changed. The impurities isolated within the anode bags are the heavy silver chloride at the bottom of the bags and the floating lighter scum or "dross", mainly of mixed silicates, and borates; and arsenic and antimony complexes in silicro-borate form. This dross can be skimmed off or separated before a silver recovery procedure. The AgCl in the bags is usually recovered after removal of the bags by complexing and dissolving with cyanide or ammonium hydroxide solutions in which the dross compounds are insoluble. Some of the electrolyte is withdrawn from the cell for purification and replaced with fresh solutions of acid and gold chloride.
The gold ions pass thru the permeable bags to deposit on the cathode.
In my previous patent, SHOR U.S. Pat. No. 4,612,093, I describe an inexpensive process and apparatus for purification of karat gold, i.e. alloyed gold usually from secondary sources. Karat gold in the jewelry trade is gold alloyed with other alloying metals for hardening, coloring or other jewelry desired qualities. In this process the karat gold is electrolyzed from karat gold anodes in an "impregnated" salt water solution against inert cathodes. The gold ions are confined away from the cathodes by a semi-permeable membrane and accumulate in the electrolyte. The confined electrolyte is periodically withdrawn from the electrolytic cell and the gold ions are selectively reduced by chemical means. As this process does not utilize or generate toxic chemicals or gases, it can be practiced in a simple apparatus. No hoods or special ventilating systems are needed. It is very useful for operation by jewelers or in small plants. Recovery of the other elements including silver, copper, zinc is provided by my process.
Another process disclosed by TRUTHE in U.S. Pat. No. 1,996,985, describes another method for purification of karat gold. TRUTHE's method involves the purification of karat gold by electrolytically leaching from the scrap anodes, the non-gold metallic (copper, zinc, silver, nickel and platinum metals). A nitrate electrolyte of Cu(NO.sub.3).sub.3 and dilute nitric acid is used for this electroleaching. The gold and platinum metals are insoluble in this leaching electrolyte and are collected in bags as anode slime. The non-gold metallics such as silver and copper are soluble in this nitrate electrolyte. Depending on the electropotential used, the silver and nickel metals deposit on the cathode and the other non-gold metals remain dissolved in the electrolyte, and if desired are recovered therefrom.
A method has also been disclosed in U.S. Pat. No. 3,663,388 for peeling pure gold from scrap gold contacts used in the electronic industry and also for masked etching and/or removal from electronic circuit boards. The gold is electrolyzed and peeled to from a non-adherent gold "oxide" in aqueous inorganic salt solutions. The non-adherent pure gold is removed from the substrate, by vibration and recovered in its pure form. No purification from alloying elements is involved as the gold used for electronic circuitry is essentially pure gold.